The invention relates to a work head for bell forming in automatic bell-forming machines, for pipe end portions made of plastic material, and in particular for pipes made of polyethylene and the like. In the manufacture of thermoplastic pipes destined for use in fluid conveyor pipelines, for example such as those used in the construction industry, bell-forming machines are employed to fashion at the pipe ends the characteristic bells which serve for connecting a plurality of individual pipes in succession, with the aim of obtaining a continuous pipeline of a desired length.
In a bell-forming machine, the pipe is associated to a special workhead which combines shaped buffer with forming chambers to heat-form the pipe to create a pipe bell in one end portion, cooling it inside the workhead in order to render the belling permanent.
Polyvinylchloride and ABS pipes and the like shrink during the above-mentioned cooling operation, and once cooled retain their shape permanently. The telescopic joining of the pipe spigots and bells thus never causes problems, even after a long period spent in a warehouse, or even after a prolonged exposition to a heat source, as happens, for example, in the case of pipes left exposed to the heat of the sun for long periods.
This dimensional and geometric stability is not at present attainable with pipes made of polyethylene, as pipes made of this material, at the end of the bell-forming process performed in a traditional bell-forming machine, remain in a state of internal tension which is apparently stable but which as soon as the pipe is subjected to solar rays or hostile thermal conditions, causes the shrinking process to recommence such that, often, the final shape and dimensions of the pipe are no longer compatible with the tolerances required for fitting.
This instability and the extreme difficulty of controlling the shrinkage of the belled end of the pipe inhibits the use of polyethylene in such applications.
Polyethylene, however, boasts provides several advantages: durability over a long period of time, weight, flexibility, and adaptability at the workplace, which overcome the disadvantage of fairly high cost, and render it a popular choice.
The prior art teaches no bell-forming machine which is capable of belling polyethylene pipes, and indeed other special equipment is used to bell-form them, all of which involve expensive constructional processes requiring the preparation of the tube and bell in separate parts, with specialized operations for welding them together when the pipe in place at a location of use.